The invention relates to a record carrier, and more particularly, to a record carrier in which a signal having a sequence of successive information signal portions is recorded in a track.
A record carrier and related methods and devices for recording are published by K. A. Schouhamer Immink in the book entitled "Coding Techniques for Digital Recorders" (ISBN 0-13-140047-9). In said title, for example, the so-called EFM modulation system is described which is used for recording information on so-called Compact Discs. The EFM-modulated signal is obtained by converting a series of 8-bit information words to a series of 14-bit code words, three merging bits being inserted into the code words. The code words are selected such that the minimum number of "0" bits situated between the "1" bits is d (2) and the maximum number is k (10). This constraint is also referenced dk-constraint. The series of code words is converted, via a modulo-2 integration operation, to a corresponding signal formed by bit cells having a high or low signal value, a "1"-bit being represented in the modulated signal by a change from the high to the low signal value or vice versa. A "0"-bit is represented by the lack of a change of signal value at a transition between two bit cells. The merging bits are selected such that even in the regions of transition between two code words the dk-constraint is satisfied and that in the corresponding signal the so-called running digital sum value remains substantially constant. The running digital sum value at a specific instant is understood to mean the difference between the number of bit cells having the high signal value and the number of bit cells having the low signal value, calculated over the modulated signal portion situated before this instant. A substantially constant running digital sum value means that the frequency spectrum of the signal does not comprise frequency components in the low frequency area. Such a signal is also referenced a DC-free signal. The lack of low-frequency components in the signal is highly advantageous when the signal is read from a record carrier on which the signal is recorded in the track, because then continuous tracking control unaffected by the recorded signal is possible. Information recording has a constant need for enhancing the information density on the record carrier.
A possible solution to this is a reduction of the number of bit cells per information word in the modulated signal. However, the problem occurring then is that as a result of the reduction of this number of bit cells per information word the number of unique bit combinations which may represent the information words will decrease, due to which less strict constraints can be imposed on the modulated signal, for example, constraints as regards low-frequency contents of the modulated signal.